Advance s in electronics technology have enabled the design and cost-effective fabrication of portable electronic devices. Thus, usage of portable electronic devices continues to increase both in the number of products available and the types of products. Examples of the broad spectrum of portable electronic devices include pagers, cellular telephones, music players, calculators, laptop computers, and personal digital assistants, as well as others.
The electronics in a portable electronic device generally require direct current (DC) electrical power. Typically, one or more batteries are use d as an energy source t o provide this DC electrical power. Ideally, the energy source would be perfectly matched to the energy requirements of the portable electronic device, However, most often the voltage and current from the batteries is unsuitable for directly powering the electronics of the portable electronic device. For example, the voltage level from the batteries may differ from the voltage level required by the device. In addition, some portions of the electronics may operate at a different voltage level than other portions, requiring different energy source voltage levels. Also, batteries are unable to respond quickly to rapid fluctuations in current demand.
The typical arrangement is shown in FIG. 1 for a portable electronic device 10 that includes an energy source 12, such as the one or more batteries, and a load device 14, such as the electronics that require electrical power. Interposed between the energy source 12 and the load device 14 is a power supply 16 that may perform a number of functions. For example, a power converter 20, depicted as integral to the power supply 16, provides the necessary changes to the power from the energy source 12 to make it suitable f or load device 14.
The power supply 16 may also perform functions other than power conversion. For example, protecting the energy source 12, load device 14 and/or power converter 20 from damage by a sustained high electrical current may require electrically disconnecting the energy source 12 from the rest of the portable electronic device 10. As another example, the power converter 20 may require assistance during start-up.
Regarding the types of power conversion required, the power converter 20 may "step up" (i.e., boost) or "step down" the voltage. That is, the converter 20 may increase or decrease the output voltage V.sub.OUT provided to the load device 14 with respect to the input voltage V.sub.S from the energy source 12. The power converter 20 may also store an amount of energy to satisfy a brief spike or increase in demand by the load device 14 that the energy source 12 is unable to provide.
The power converter 20 may also regulate the output voltage V.sub.OUT, keeping it close to the desired output voltage level and reducing rapid fluctuations that may cause detrimental noise or cause undesirable performance of the load device 14. Such fluctuations may occur due to changes in demand, induced noise from external electromagnetic sources, characteristics of the energy source 12, and/or noise from other components in the power supply 16.
Although power converters 20 provide many benefits, existing power converters 20 also place undesirable performance constraints on portable electronic devices 10. The specific attributes of generally known power converters 20 are discussed below along with the types of constraints generally encountered.
Many generally known power converters 20 are optimized for a specific energy source 12 and a specific load demand from the load device 14. The power converter 20 may not accommodate, or only accommodate inefficiently, variations in the voltage and current characteristics of the energy source 12 and/or the load device 14. For example, some types of power converters 20 cannot provide an output voltage V.sub.OUT that is higher than the input voltage V.sub.S and/or their efficiency is related to how close the input voltage V.sub.S is to the required output voltage V.sub.OUT. In addition, some power converters 20 are incapable of providing medium power levels such as 0.5-1.0 W. Moreover, generally known power converters 20 have a design that will only operate within a narrow range of input voltages, output voltages and power capacities.
Additionally, as will be discussed below with regard to FIG. 2, some power converters 20 achieve an acceptably regulated output voltage V.sub.OUT only through inefficient voltage regulators.
In other instances, voltage regulation by the power converter 20 is inadequate for the needs of the load device 14. For example, the nominal output voltage V.sub.OUT may vary due to variations in the input voltage V.sub.S, variations in the temperature of the power converter or the output current drawn by the load device 14. Also, even if V.sub.OUT is at an acceptable nominal output level, the power converter 20 may undesirably oscillate about the nominal output voltage V.sub.OUT. This voltage ripple V.sub.RIP is defined as the range of the oscillations about the nominal output voltage V.sub.OUT and may impair or preclude proper operation of the load device 14.
Therefore, existing power converters 20 do not efficiently provide on demand the required power to a load device, nor adjust to variations in the energy source and load device to provide a stable V.sub.OUT.
Furthermore, existing power converters 20 do not operate with low input voltage levels, such as a sub-one volt input voltage V.sub.S. The existing power converters 20 usually require an operational bias voltage that is typically comparable to the output voltage demands of the load device 14, which a re generally greater than one volt. Also, a certain amount of noise is superimposed on the input voltage V.sub.S by external and internal sources. When the input voltage level V.sub.S is low, this noise may become relatively significant, degrading or precluding operation of the power converter 20.
One implication of requiring an input voltage of greater than one volt is that an otherwise desirable single cell battery, or an alternative source of power, may be inappropriate as an energy source 12 for the device 10. For example, the nominal voltage supplied by certain electrochemical batteries or alternative sources of power may be below one volt, or have a voltage characteristic that decreases as their stored charge decreases. Such batteries have a significant amount, and perhaps a majority of, their stored energy, which is retrievable only at a sub-one volt level. Consequently, the service life of the battery in a portable electronic device 10 is limited by the inability of the device to operate with a sub-one volt input voltage V.sub.S from the battery. As a result, batteries are discarded with a significant amount of charge or "life" still left in them. Achieving additional service life by incorporating additional batteries into the device 10 increases the size and weight of the device 10.
Therefore, many existing power converters do not operate (or operate desirably) with a sub-one volt input voltage.
Furthermore, even if a power converter 20 can continuously operate at a sub-one volt input voltage V.sub.S, generally a higher input voltage level (i.e. over 1 volt) is required to start the power converter 20. Th at is, the converter requires a higher input voltage at the start-up phase than is necessary for continuous operation (e.g., 0.4 V higher). Therefore, the power converter 20 must be continuously operated once the minimum start-up input voltage is reached, thus consuming power, in order to enhance the amount of energy that is retrieved from the energy source 12.
For the start-up phase, an external start-up circuit (such as a Schottky diode) is often added to the existing power converters 20. The start-up circuit assists in overcoming the additional input voltage requirement at start-up and in shortening the period of time required for the power converter 20 to reach its designed output voltage. However, generally known start-up circuits are usually not capable of operating at sub-one volt input voltage. Also, having to use an external start-up circuit limits the ability to miniaturize the power converter 20. In addition, external start-up circuits tend to dissipate power even when the power converter 20 is not in a start-up condition, thereby reducing the efficiency of the power converter 20.
Therefore, existing power converters 20 are generally incapable of starting with a sub-one volt input voltage, nor efficiently provide for start-up with a greater than one volt input voltage.
Another drawback to the existing power converters 20 is that they cannot efficiently provide the output voltage required with sub-micron integrated circuits. Integrated circuit design in portable electronic devices 10 is moving toward circuits with lower operating voltages. For example, current fabrication capabilities for Complementary Metal Oxide Semiconductor (CMOS) based on sub-micron technologies (0.5 .mu.m and less) typically provide for devices that operate at 3.0-3.3 V. The projected technology development for reducing the feature size of such integrated circuits will mean further reducing this operating voltage, and thus power supplies and power converters will have to be developed for providing these reduced operating voltages.
For example, trends in microprocessor design highlight the need and the advantages of power supplies that operate at the lower operating voltages. The feature size of the integrated circuit components of a microprocessor is reduced to increase functionality at a reduced cost. Thus, one chip may contain the circuitry of a number of chips and discrete components. Smaller feature size also allows for the microprocessor to perform its functions more quickly. With smaller features, digital switching may be performed more quickly. Since switched components tend to generate heat in proportion to the rate in which they are switched, more densely packed and more quickly switched components make heat dissipation a limiting constraint on the design of the microprocessor. The increased switching also means that each feature may act as a Radio Frequency (RF) antenna, emitting Electromagnetic Interference (EMI) to adjacent features. Reducing the operating voltage of the microprocessor accommodates the reduction in feature size, the increased switching and the heat dissipation. Still further, as mentioned, the heat generated by the features is typically proportional to the operating frequency; however, the heat generated is also quadratically related to the operating voltage, that is, reducing the operating voltage by half reduces the heat generated to a quarter. Thus, the resultant trend in lower operating voltage can be seen by typical microprocessors using 5 V in 1990, 3.3 V in 1995, 1.8-2.4 V in 1998, 1.2-2.4 V in 2000, and 1 V or less expected thereafter.
As the feature size becomes smaller, the current carrying capability of each feature is also reduced. Consequently, a lower operating voltage provides for reducing this current so that the feature does not fail.
Still further, the distance between features is reduced, and thus the amount of insulating material between the features is reduced. Consequently, a lower operating voltage avoids a breakdown through the thinner insulating material between features that would cause microprocessor failure.
Therefore, a significant need exists for a power converter that may provide an output voltage V.sub.OUT that addresses the lower operating voltages required by smaller and faster integrated circuits and microprocessors. More particularly, it is desirable that the power converter may advantageously generate regulated output voltage V.sub.OUT in the range of 0.8-1.6V.
Still another drawback to existing power converters 20 is that they are not suitable for the desired level of miniaturization in portable devices and are not appropriate for embedded applications, even considering integrated circuit structures such as Silicon On Insulator (SOI) and Silicon On Metal (SOM). In some instances, miniaturization is not possible due to the number of discrete external components necessary that are not amenable to integrated circuit fabrication. These components thus require a printed circuit board (PCB), hybrid or multi-chip module (MCM) design wherein the size and expense of such fabrication is higher than for an entirely integrated circuit.
In addition, the efficiency of generally known power converters 20 results in an amount of heat generation that is not suitable for further miniaturization.
Therefore, existing power converters 20 cannot be fabricated as an integrated circuit, especially embedded with a load device 14.
Another drawback to conventional power converters is that they may emit undesirable amounts of electromagnetic interference (EMI) that must be controlled by distancing and/or shielding the load device 14. The EMI may be from an inductor incorporated into the power converter 20, or the result of reducing the feature size of the circuits in the power converter 20. When seeking to reduce the size of discrete components through use of smaller components, the energy storage and transfer capability is necessarily also reduced. Therefore, a higher operating frequency is required to transfer an equivalent amount of power. However, the higher operating frequency also results in EMI that is detrimental to the portable electronic device 10. Moreover, the portable electronic device 10 itself generally has federally mandated limits on RF emissions that may be exceeded by a sufficiently high operating frequency.
Therefore, it is also desirable that the power converter 20 should advantageously generate minimal amounts of heat or radiated energy (EMI) to the load device 14, and thus be suitable for being embedded on the same integrated circuit or module.
Therefore, various of the existing types of power converters 20 are inappropriate for addressing one or more of the drawbacks noted above and for satisfying needs in the industry and market place. As such, it is desirable to improve upon the power converter technology to address various of the drawback noted above.